1. Field of the Invention
The present invention relates to a turbine moving blade assembly, which is particularly used for a portion from an intermediate pressure section to a low pressure section in a steam turbine and the like, and to a turbine having such turbine moving blade assembly, and more particularly, relates to a turbine moving blade assembly for suppressing vibration of respective turbine moving blades and a turbine having the turbine moving blade assembly.
The turbine blade assembly is composed of a plurality of turbine moving blades mounted, in a circumferential direction, on a rotor of a turbine and including twisted blades each having a relatively long blade length and being twisted from root potions toward tip portion.
2. Description of the Related Art
Recently, it is strongly required for many power generation plants to be operated at a high loads with a high availability regardless of their types. Thus, a turbine as main equipment of a power generation plant must withstand the operation at part load as well as rated load, and the repeated start and stop with the significant change in operation. Accordingly, sufficient reliability for operation is further required than ever to all the elements or components constituting the turbine.
A turbine moving blade, in particular, a final stage turbine blade having a long length, which is subjected to a large centrifugal force, is typically exemplified as a most important component of the turbine components described above.
An important problem of the operation reliability of a turbine moving blade, in particular, a long blade, resides in how to suppress a resonant phenomenon under the condition that an excitation frequency the rotor speed coincides with one of the natural frequencies of the turbine moving blades.
In a case of the turbine moving blades, in particular, a long blade, composed of twisted airfoils, a twist/untwist (hereinafter, also called untwist) force acts thereon increases. Various shapes of snubber blades enhance vibration suppression effect by changing a vibration mode by coupling the turbine moving blades with each other at the rated rotor speed of a turbine making use of the untwist force, and such snubber blades are widely used as shown in FIGS. 16 and 17.
As shown in FIG. 16, a plurality of turbine moving blades 1, 2, 3 . . . are arranged and assembled in a circumferential direction of a turbine rotor 4. These turbine moving blade 1, 2, 3 . . . are twisted blades, each having an airfoil portion 5 twisted from a root portion 5b toward a tip portion 5a in its sectional shape.
Shrouds 6 are formed to the tip portion 5a of the airfoil portion 5 (i.e., blade tip portion 1a, 2a . . . of FIG. 17) in the turbine moving blades 1, 2 . . . so as to be integral therewith, respectively. As shown in FIG. 17, the respective shrouds 6 of the turbine moving blades 1, 2, . . . have leading side snubbers 1b, 2b . . . projecting from the leading edge suction side of the blade tip portion 1a, 2a . . . and trailing side snubbers 1c, 2c, . . . projecting from the trailing edge pressure side of the blade tip portion 1a, 2a . . . . 
A turbine moving blade coupling, which can couple the turbine moving blades 1, 2 . . . with each other, is composed of a plurality of shrouds 6 having the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c . . . , respectively.
Further, it is to be noted that a reason why the shrouds 6 are not formed to cover entire airfoil shape at tip resides in that a centrifugal force acting on the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c . . . is reduced by minimizing the volume of the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c. 
When the turbine moving blades 1, 2 . . . are assembled, that is, in a turbine stop state of a portion of FIG. 17A, a gap (assembly gap) D is set between, for example, the trailing side snubber 1c of the turbine moving blade 1 and the leading side snubber 2b of the turbine moving blade 2.
When the centrifugal force acting on the turbine moving blades 1, 2 . . . increases as the rotor speed of the turbine increases, since an untwist force acts on the airfoil portions 5 of the turbine moving blades 1, 2 . . . , a gap D (gap caused in assembling) is gradually narrowed, and a contact face 1f of the trailing side snubber 1c starts to come into contact with a contact face 2f of the leading side snubber 2b at a specific rotor speed Rc for start of contact, thus giving a state as shown in FIG. 17B.
When the contact face if once comes into contact with the contact face 2f, even if the rotor speed of the turbine further increases, the relative position of the trailing side snubber 1c and the leading side snubber 2b does not change, and a reaction force between the contact faces 1f and 2f increases.
When the gap D between the leading side snubber 2b and the trailing side snubber 1c is too large, the contact face 1f does not come into contact with the contact face 2f even if a predetermined untwist force acts thereon, and consequently, a vibration suppression effect is not attained at the rated rotor speed of the turbine. On the contrary, when the gap D is too small, the contact reaction force is made too large, an excessive stress occurs at the root portion in which the leading side snubber 1b or the trailing side snubber 1c projects from the blade tip portion 1a of the turbine moving blade 1.
Accordingly, the contact start rotor speed Rc, at which the leading side snubber 2b starts to come contact with the trailing side snubber 1c, must be determined in consideration of the magnitude of the contact reaction force acting on the contact faces 1f and 2f at the rated speed or an over speed of the turbine, in particular, in consideration of the magnitude of the stress in the root portions in which the leading side snubbers 1b, 2b . . . and the trailing side snubber 1c, 2c . . . project from blade tip portion 1a, 2a . . . in view of strength.
When the contact start rotor speed Rc is set to a specific value, the untwist of the turbine moving blades 1, 2 . . . after, for example, the trailing side snubber 1c came into contact with the leading side snubber 2b, holds a constant value at the rotor speeds higher than the contact start rotor speed Rc as shown in a curve 101 (shown by a solid line) with respect to a curve 100 (shown by a broken line) of a single blade as shown in FIG. 18, and the contact reaction force increases as the rotor speed increases at the rotor speeds higher than the contact start rotor speed Rc as shown in a curve 102.
FIG. 19 shows an example of a Campbell diagram of the turbine moving blades 1, 2 . . . as described above and shows the relationship between the change in a natural frequency of the turbine moving blades 1, 2 . . . (single blade mode, continuously-coupled blade mode) and the rotor speed of the turbine, with the reference of multiple frequencies of rotor speed.
When, for example, a letter T shows a natural frequency of a vibration mode in a tangential direction (turbine rotating direction) that is a fundamental mode of the single turbine moving blade, the natural frequency of the vibration mode T resonates at t1 with the double-speed component, at t2 with the triple-speed component, and at t3 with the quadruple-speed component of the turbine, and there is a possibility that the vibration stress increases at these resonant points. Further, when a letter A shows a natural frequency of a vibration mode in an axial direction (turbine axial direction) that is also a fundamental mode of the single turbine moving blade, the natural frequency resonates at a1 with the quadruple-speed component, and at a2 with the triple-speed component of the turbine.
Whether or not an operation at these resonant points t1, t2, t3, a1, a2 is dangerous depends on the magnitude of the excitation force and the vibration response characteristics of the turbine moving blades 1, 2 . . . at these resonant points. In general, an excitation force is high for lower values of multiples and for higher rotor speed, and the vibration response is higher for the lower modes of vibration.
Further, in the turbine moving blades 1, 2 . . . , when the rotor speed of the turbine exceeds the contact start speed Rc of the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c . . . , the vibration mode of the turbine moving blades 1, 2 . . . shift from the single blade mode to the continuously-coupled mode. Since the vibration of the continuously-coupled mode is an axial vibration GA mode as a blade group, the vibration level thereof becomes low at the resonant points, besides the natural frequency of the axial vibration GA mode is sufficiently separated from the multiples of rated rotor Ro. Accordingly, the vibration of the continuously-coupled turbine moving blades 1, 2 . . . is suppressed.
Japanese Unexamined Patent Application Publication No. H02-16303 (Patent Publication 1) discloses a turbine moving blade coupling for shifting, when the vibration level is high at any of the resonant points a1, a2, t1, t2, t3 in the single blade mode shown in FIG. 19, the single blade mode in lower speed range to the continuously-coupled mode to suppress the increasing in vibration. This structure is shown in FIGS. 20 to 22, which mainly employs a technology for shifting to the continuous coupling of the adjacent turbine moving blades 1, 2 . . . by the leading side snubbers 1b, 2b, . . . and the trailing side snubbers 1c, 2c . . . making use of the untwist force in a higher speed range. However, the technology realizes the continuous coupling in the lower speed range as well as in the higher speed range by changing the contact faces of snubbers capable of coming into contact even in the lower speed range.
When it is intended to realize the contact from the lower speed range to the higher speed range by one contact face, the contact start rotor speed Rc is set to the lower speed range as apparent from the contact reaction force characteristic curve 102 of FIG. 18. However, in this case, since the contact reaction force in the higher speed range becomes too large, the strength of the root portions, in which the leading side snubbers 1b, 2b and the trailing side snubbers 1c, 2c . . . are projected from the blade tip portion 1a, 1b . . . of the turbine moving blades 1, 2 . . . , is deteriorated.
To cope with the above problem, the Patent Publication 1 employs a system for providing steps on the contact faces 1f, 1g, 2f, 2g . . . of the trailing side snubbers 1c, 2c . . . , for causing, for example, the contact face 1g of the trailing side snubber 1c to come into contact with the contact face 2f of the leading side snubber 2b in the turbine lower speed range, and for causing, for example, the contact face 1f of the trailing side snubber 1c to come into contact with the contact face 2f of the leading side snubber 2b in the turbine higher speed range as shown in FIG. 20.
FIG. 21 shows a system for replacing the positions of the contact faces having the steps with a case shown in FIG. 20, causing, for example, the contact face 1f of the trailing side snubber 1c to come into contact with the contact face 2g of the leading side snubber 2b in the turbine lower speed range, and causing, for example, the contact face 1f of the trailing side snubber 1c to come into contact with the contact face 2f of the leading side snubber 2b in the higher speed range.
Further, FIG. 22 shows a system for providing a projection 1m to one of the contact face (for example, the contact face 1f of the trailing side snubber 1c) in place of the step so that the projection 1m comes into contact with the other contact face (for example, the contact face 2f of the leading side snubber 2b) in the lower speed range.
In the arrangement mentioned above, when the gap between the contact faces is appropriately selected, the contact reaction force on each contact faces (projection) is made as shown in FIG. 23 when the contact reaction force characteristic curve 102 of FIG. 18 is used as a reference. That is, the contact starts at a rotor speed of r1 according to a contact reaction force characteristic curve 103 and separated at a rotor speed of r2.
When the rotor speed further increases, the contacting starts again at the rotor speed of Rc according to a contact reaction force characteristic curve 104 and keeps the contacting state over the rated rotor speed of Ro with the contact reaction force increasing.
As described above, the blades take continuously-coupled mode in the rotor speed range in which any of the faces contact.
Further, it is to be noted that a part of the single blade mode in the lower speed range shown in the Campbell diagram of FIG. 19 is replaced by the continuously-coupled mode as shown in FIG. 24 for the turbine moving blades 1, 2 . . . shown in FIGS. 20 to 22.
It may be found from the above explanation that basic requirement for improving the reliability of the turbine moving blades 1, 2 . . . employing the snubbers is approximately satisfied by the conventional (background) technology.
That is, first, the leading side snubbers 1b, 2b, . . . and the trailing side snubbers 1c, 2c . . . are formed such that the amount of projection thereof projecting from the blade tip portion 1a, 2a . . . is further reduced as the blades are longer to improve safety by suppressing a centrifugal force.
Second, when the gap (assembly gap) between the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c . . . is reduced at the time when the turbine is stopped, the leading side snubbers 1b, 2b . . . can be caused to come into contact with the trailing side snubbers 1c, 2c . . . in the lower speed range. However, since the contact reaction force becomes too large when the rated rotor speed of the turbine is reached, consequently, the stress becomes too large in the root portions in which the snubbers project from the blade tip portion 1a, 2a . . . , an assembly gap of an appropriate value is set.
Third, in order to obtain the continuously-coupled instead of the single blade mode in the lower speed range, an additional contact face (projection), which makes contact even in the lower speed range, is provided to a regular contact face in the higher speed range.
Actually, however, there are scatters as to the assembled state of the turbine moving blades, the untwist force of the airfoil portions 5 of the turbine moving blades 1, 2 . . . , and the contact between the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c . . . , and the like. As a result, there are scatters in the rotor speed at which contact starts, a contact area, and the like. Accordingly, in order to improve the reliability of the turbine moving blades 1, 2 . . . , it is necessary to consider the reduction of the adverse affect due to the scatter mentioned above as well as the basic requirements also mentioned above.
Here, a consideration will be made on a case, in which the above-mentioned scatter occurs to the gap D between the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c . . . of the turbine moving blades 1, 2 . . . shown in FIG. 17. In this case, first, it is assumed as shown in FIG. 25 that the gap D is different because the turbine moving blade 2 is assembled by being slightly inclined in a turbine rotating direction, i.e., to the turbine moving blade 1 side.
In this case, since a gap D2 between the trailing side snubber 2c of the turbine moving blade 2 and the leading side snubber 3b of the turbine moving blade 3 is larger than a gap D1 between the trailing side snubbers 1c of the turbine moving blade 1 and the leading side snubber 2b of the turbine moving blade 2, i.e., D2>D0>D1, wherein D0 is a designed assembly gap.
When the rotor speed of the turbine is increased in the above-mentioned state, the trailing side snubber 1c and the leading side snubber 2b, by which the gap D1 is formed, begin, first, to come into contact with each other (the gap D1=0), and the trailing side snubber 2c and the leading side snubber 3b, by which the gap D2 is formed, have a gap D2′(<D2) as shown in FIG. 26. As the rotor speed increases, the contact reaction force on the contact face between the trailing side snubber 1c on the gap D1 side and the front snubber 2b is increased until the trailing side snubber 2c starts to come into contact with the leading side snubber 3b. 
In this state, a contact reaction force Fc acts on the contact face 2f of the leading side snubber 3b of the turbine moving blade 2 from the contact face 1f of the trailing side snubber 1c of the turbine moving blade 1 in a direction vertical to the contact face 2f. 
An axial component Fa of the turbine rotor (rotor) in the contact reaction force Fc acts in a direction in which the turbine moving blade 2 is inclined towards outlet side in the axial direction of the rotor thereof.
Furthermore, when the turbine moving blade 2 is assembled in a counter-rotating direction of the turbine, i.e., assembled by being slightly inclined on the turbine moving blade 3 side contrary to the example shown in FIGS. 25 and 26, and the gap D is different as shown “D1>D0>D2”, the trailing side snubber 2c on the gap D2 side first starts to come into contact with the leading side snubber 3b. In this case, the contact reaction force from the turbine moving blade 3 acts on the contact face 2f of the trailing side snubber 2c of the turbine moving blade 2 vertically to the contact face 2f. 
An axial component of the rotor in the contact reaction force acts in a direction in which the turbine moving blade 2 is inclined towards the inlet side of the axial direction of the rotor.
That is, when there is scatter in the assembly gaps D between the leading side snubbers 1b, 2b . . . and the trailing side snubber 1c, 2c, . . . of the turbine moving blades 1, 2 . . . , the contact start rotor speed Rc in the respective turbine moving blades 1, 2 . . . is scattered. As a result, some of the turbine moving blades 1, 2 . . . in rotation are inclined towards the inlet side (front side in the turbine rotor axial direction) or towards the outlet side (rear side in turbine rotor axial direction).
As described above when the turbine moving blades 1, 2 . . . are restricted once, due to the friction force on the contact faces of the leading side snubbers 1b, 2b . . . and the trailing side snubbers 1c, 2c . . . it is very difficult to change the relative position therebetween, and thus, the restricted position therebetween is kept as it is until the turbine is operated at a rated speed. As a result, when the turbine moving blades 1, 2 . . . are outstandingly inclined towards the inlet side or to the outlet side with respect to the rotor, not only the performance of the blade row is adversely affected but also the turbine moving blades 1, 2 . . . suffer from erosion in different degree, and thus, there is a possibility that a turbine performance as well as the blade reliability is deteriorated.